It is desirable to test all cells in a liquid crystal display (LCD) prior. to assembling the array into a Liquid Crystal Display Panel. This eliminates alignment, assembly and filling costs for rejected arrays. A cell normally comprises a thin film transistor (TFT), and a pad which may be modeled electrically as one side of a capacitor. Thus, electrically, the cell appears to be a TFT with a series cell capacitance attached to its source. Since, it is extremely expensive and time consuming to test each cell in the array, individual cells, located at test sights on the border of the array are normally used to conduct preassembly tests. Because this test does not check for bus continuity, individual transistor functionality, or contact to the indium-tin-oxide (ITO) pad, arrays that pass the this type of preassembly test may still fail to produce acceptable displays. It would, therefore, be advantageous to provide a method and apparatus for quickly and economically testing each cell in the array to assure that it is operational. It would also be advantageous if the method and apparatus provided information on the opcrational characteristics of individual cells. Normally, when testing prior to assembly, the front plate of the display is not in place, which reduces the cell capacitance and makes measuring charge stored thereon more difficult.
Conventional methods of testing individual elements of the array involve mechanically probing the array. For example, the source and drain pads may be accessed using a mechanical probe. Once the source and drain pads are accessed, the status of individual TFT's may be determined. Unfortunately, this method of testing is extremely time consuming and may physically damage the array. Therefore, testing of display cell transistors is presently done of experimental, small area arrays using mechanically positioned probes. It would, therefore, be advantageous to test all transistors, ITO contacts, gate and drain lines on a large array without the front plate (counterelectrode) in place. It would further be advantageous to test each individual element of the array and obtain useful analog information without physically contacting any portion of the cell being tested.